1. Field of the Invention
The present invention relates to an apparatus and a method for generating a Fourier spectrum for a test object in Fourier transform spectrographs.
2. Description of the Prior Art
FIG. 1 shows a schematic view of a prior art Fourier transform infrared spectrograph. Infrared rays are emitted from a light source 1. A collimating mirror 2 converts the infrared rays into collimated infrared light beams 3 which is incident on an infrared beam splitter 4. Infrared beam splitter 4 splits the infrared light beams 3 into two light beams. The two split beams are reflected by a movable reflecting mirror 6 and a fixed reflection mirror 5 respectively, and recombined in infrared beam splitter 4 to interfere with each other, and proceeds to a condenser mirror 7. A corner cube mirror is used for a fixed reflecting mirror 5 and movable reflecting mirror 6, but a plane mirror instead of the corner cube mirror may also be used.
An actuation device 8 moves movable reflecting mirror 6 forward and backward at a predetermined speed along the optical axis of infrared light beams 3 whereby recombined infrared light beam portions at splitter 4 interfere with an arbitrary optical path difference .delta..
A laser interferometer is incorporated in the spectrograph so that a change in optical path difference .delta. is measured in units of a wavelength of a laser beam 10 output from laser oscillator 9 by interference of the laser beam 10 . Laser beam 10 is split into two light beams by a visible beam splitter 11 whose split surface is on the same plane of the split surface of the infrared beam splitter 4, reflected by fixed reflecting mirror 5 and movable reflecting mirror 6 to be recombined to interfere in a visible beam splitter 12, and detected in a laser detector 13. A condenser mirror 7 irradiates a sample 14 with infrared rays and a light collecting mirror 15 collects the light transmitted through sample 14. An infrared detector 16 receives the transmitted light and converts it into an electric signal. Similarly, a laser detector 13 detects a laser interference signal 17. An infrared interference signal 18 which is detected by infrared detector 16 and laser interference signal 17 which is detected by laser detector 13 are individually input to an A/D conversion board 19.
In A/D conversion board 19, infrared interference signal 18 is sampled in connection with laser interference signal 17 which is binary processed. The sampled infrared interference signal is transferred, after A/D conversion, to a computer 20 as a digital infrared interference signal 21. Computer 20 processes data representing digital infrared interference signal 21 and laser interference signal in accordance with a predetermined procedure for spectral analysis. Computer 20 has input/output units 22.
FIG. 2 shows a circuit diagram of A/D conversion board 19 of FIG. 1. Infrared interference signal 18 is amplified by an amplifier 23, and then is sampled-and-held in a sample-and-hold circuit 24. Laser interference signal 17 is amplified by an amplifier 25. Then, a low-frequency cutoff filter 26 removes the direct current component of an amplified laser interference signal which is then converted to binary form by a comparator 27. In response to the binary laser interference signal 28, a flip-flop 29 outputs a sample-and-hold control signal 30 to a sample-and-hold circuit 24 to sample-and hold an amplified infrared interference signal. Therefore, the amplified infrared interference signal is sampled and held in coordinated relations with respect to laser interference signal 17.
An A/D converter 31 converts the sampled, held and amplified infrared interference signal to digital data 32 and digital data 32 is processed sequentially by an interface circuit 33 to be a digital infrared interference signal 21 which is in turn input to computer 20. When A/D converter 31 completes the conversion, A/D converter 31 outputs a reset signal 34 to flip-flop 29 to suspend sample-holding operation.
FIG. 3 shows a timing chart for the circuit of FIG. 2. The period of sample-and-hold control signal 30 corresponds to one wavelength, .lambda., of laser interference signal 17.
FIG. 4 shows a flow chart for data processing for computer 20 shown in FIG. 1. Computer 20 reads digital infrared interference signal 21 and obtains a power spectrum by a Fourier analysis. If the wavenumber of infrared is represented by v, an optical path difference .delta., and digital infrared interference data f(.delta.), then power spectrum P(v) corresponding to the irradiated sample 14 is given by EQU P(v)=[F.sub.s.sup.-1 (f(.delta.))].sup.2 +[Fc.sup.-1 (f(.delta.))].sup.2
where F.sub.s.sup.-1 represents the sine component of inverse Fourier transform and Fc.sup.-1 represents the cosine component of inverse Fourier transform.
Digital data corresponding to infrared interference signal 21 are subjected to spectral computation and display after the data is accumulated for a predetermined number of times in order to improve a signal to noise (S/N) ratio. Since the S/N ratio increases proportional in general to the square root of the number of times of accumulations, an increase in the number of times of accumulations is not very effective on improvement of the S/N ratio in spite of an increase in measurement time. Moreover, there arise problems such as instability and temperature change of a sample and an apparatus with time, so that the number of times of accumulations is limited to an appropriate number.
With recent advancements in high technology, demands on the accuracy of the analysis tend to increase, particularly analyzing under more severe conditions such as analyzing a minute amount of sample, thin film and surface conditions and a transient phenomena. Therefore, reducing the measurement time and processing the data at a high speed in Fourier transform spectrographs has become increasingly important.
A simple method for reducing the measurement time of the data is increasing the number of scans per unit time of movable reflecting mirror 6 by increasing a moving speed of actuation device 8 for movable reflecting mirror 6 shown in FIG. 1. However, when the moving speed of movable reflecting mirror 6 is increased, the frequency of infrared interference signal 18 is proportionally increased to the extent that the response speed of infrared detector 16 is not sufficient to follow, and therefore, detection sensitivity is decreased.
For example, in FIG. 5, timing charts (a) and (b) show laser interference signal 17 and infrared interference signal 18, respectively, at a given moving speed of movable reflecting mirror 6. Timing charts (c) and (d) show laser interference signal 17 and corresponding infrared interference signal 18, respectively, when the moving speed of movable reflecting mirror 6 is increased to twice that corresponding to timing chart (a). It is easily understood that detection sensitivity of infrared interference signal 18 corresponding to timing chart (d) is substantially decreased compared to that corresponding to timing chart (b). Further, although noise per unit bandwidth of the infrared interference signal is decreased, as its the frequency is increased, noise equivalent power (NEP) is increased, thus deteriorating the S/N ratio.
The maximum operating frequency of a collector-type detector used most widely for infrared detector 16 is about 5 kHz, although A/D converters with a conversion speed of about 1 MHz are readily available commercially. Therefore, the measurement time is limited by the operating frequency or response speed of infrared detector 16, although reducing the measurement time is critical in obtaining an accurate spectrum analysis.